Simple interest: the flat-rate math behind short loans
Not every loan charges interest on its own interest. Simple interest charges a flat percentage of the original amount for as long as the loan runs — no compounding, no interest-on-interest, just a straight line.
The formula
Interest = principal × (rate ÷ 100) × years. The principal itself never changes during the loan; only the interest owed grows, and it grows by the same amount every year.
A real example
Take an $8,000 personal loan at a flat 6% annual rate for 3 years.
- Interest:
8,000 × 0.06 × 3 = $1,440. - Total to repay:
8,000 + 1,440 = $9,440.
That $1,440 is fixed the moment the loan is signed — three years always produce the same interest at this rate, because simple interest never charges interest on interest that's already accrued.
A shorter loan
Take a $3,000 loan at 9% for 6 months — half a year.
- Interest:
3,000 × 0.09 × 0.5 = $135. - Total:
3,000 + 135 = $3,135.
"Years" just needs to be a fraction of a year for a loan under 12 months — 6 months is 0.5, 3 months would be 0.25.
Simple vs compound: same rate, same time, different outcome
Run the same $8,000-at-6%-for-3-years numbers through annual compounding instead: 8,000 × 1.06³ = 8,000 × 1.191016 ≈ $9,528.13. That's 9,528.13 − 9,440.00 = $88.13 more than simple interest produced on the identical principal, rate and term — the gap is entirely from compounding charging interest on the first year's interest once year two starts, which simple interest never does.
Where simple interest actually shows up
Most everyday loans — mortgages, credit cards, most auto loans — compound. Simple interest tends to show up in short-term loans, some promotional financing, certain personal loans, and bonds, where flat, predictable math is the point. The label on a loan doesn't always spell out "simple" or "compound," so it's worth asking directly before assuming.
Checking a lender's math
The formula also works as a sanity check. If a lender quotes a $10,000, 4-year loan at 7% simple interest, the interest should come to 10,000 × 0.07 × 4 = $2,800, for a total of $12,800. If the payoff figure you're actually quoted is meaningfully higher than that, ask what else is in it — a fee folded into the balance, or a rate that isn't really simple interest despite being called that. The formula has no hidden terms; if the total doesn't match it, something outside the formula is adding to the bill.
What this doesn't capture
This is the stated interest rate only. It leaves out origination fees, which raise the real cost of borrowing above what the rate alone suggests, and it assumes the principal sits untouched until the end — a loan actually paid down in installments along the way owes less total interest than this lump-sum formula implies.
Run your own numbers
The free Simple Interest calculator runs this instantly for your principal, rate and term — entirely on your device.